1. Field of the Invention
This invention relates to a heat-sensitive transfer recording method which can give a transfer recorded image of good printing quality even on a medium for recording having poor surface smoothness.
2. Related Background Art
The heat-transfer recording method has specific features of requiring no processed paper of chromogenic type and also having excellent durability of recorded image in addition to the general specific features of the heat-sensitive transfer recording method of light and compact device used, no noise and excellent operability and maintenance, and has been widely used in recent years.
The heat-sensitive transfer recording method uses a heat-sensitive transfer material comprising a heat transferable ink layer having a colorant dispersed in a heat-fusible binder provided by coating on a supporting member which is generally shaped in a sheet, superposes the heat-sensitive transfer material on a medium for recording so that the heat transferable ink layer may contact the medium for recording and transfers the melted ink layer onto the medium for recording by supplying heat from a thermal head from the side of the supporting member, thereby forming a transfer recorded image corresponding to the shape of heat supply (pattern) on the medium for recording.
However, according to the heat-sensitive transfer recording method of the prior art, the transfer recording performance, namely the printing quality, is greatly influenced by the surface smoothness of the medium for recording and therefore, although good printing can be effected on a medium for recording with high smoothness, there is involved the problem that printing quality will be markedly lowered in the case of a medium for recording with low smoothness. For this reason, a paper with high surface smoothness is generally used as the recording medium. However, papers with high smoothness are rather special, and ordinary papers will have various sizes of unevenness formed by entanglement of fibers. Accordingly, in the case of a paper with large surface unevenness, the thermally fused ink cannot penetrate into the fibers of paper during printing, but the ink is attached only at the convexity or in the vicinity thereof, whereby printing quality may be lowered because the edge portion of the printed image is not sharp or a part of image is defective.
In the prior art, in order to obtain a recorded image of good printing quality on such a medium for recording with poor surface smoothness, there has been employed a method based on the idea of permitting the fused ink to be attached to or to penetrate faithfully even the fine uneven structure of the medium for recording such as paper, by, for example, using a heat-fusible binder with small melt viscosity at least in the surface layer or increasing the layer thickness of the heat-transferable ink layer. However, when a binder with small melt viscosity is used, the ink layer is tacky also at relatively lower temperature to cause inconveniences such as lowering of adhesiveness and storability and contamination at the non-printed portion of the medium for recording, and also blurring of transferred image may occur. On the other hand, when the transferable ink layer is made to have a large layer thickness, blurring becomes greater and also the amount of heat supplied from the thermal head is required to be increased with the result that printing speed will be lowered.